U.S. Pat. No. 5,737,035, dated Apr. 7, 1998, shows a tuner circuit in which the Video Detector (VDET) requires a highly linear mixer to convert the second IF to baseband frequencies. The input to the mixer comes from a Variable Gain Amplifier, which buffers signals from the Second IF Filter. Its output supplies a buffer to drive a Video Sound Trap. Its Third-Order Input Intercept Point (IIP3) requirement, which is an indicator of linearity, should be high, ideally on the order of 80 dBmV or more. Moreover, signal amplitude in this path will be high to maximize the Signal-to-Noise Ratio (on the order of 60 dBmV). This precludes the use of any circuitry which would limit headroom for the signal.
The mixer itself consists of a transconductance (gm) stage and a mixing stage formed by a bipolar transistor pair. The gm stage is where most distortion occurs, and many methods have been proposed to overcome this problem, not only for mixers, but also for amplifiers.
The nonlinearity of the transconductance of each transistor comes from the fact that the gain is not constant over a range of input voltages. This nonlinearity is manifested in a change (.DELTA.) in voltage across the input of the transistor to its output. For a bipolar transistor, this is from the base to the emitter. For a MOS transistor, this is from the gate to the source.
A common linearization scheme used in this instance is the "multi-tanh" method. An example of this method can be found in "The Multi-Tanh Principle--A Tutorial Overview," IEEE J. Solid-State Circuits, Vol. 33, pp. 2-17, January, 1998. This scheme uses transistors biased at different levels to form a piecewise-linear amplifier. In effect it is several amplifiers in parallel which operate at select input offsets. Each amplifier's transfer curve overlaps its neighbor's to form a smooth response. Unfortunately, this can supply only a limited signal handling capability for a given amount of power, and requires multiple bias voltages.
Accordingly, a need exists in the art for a highly linear mixer capable of handling a wide range of signal amplitudes with a minimum of complexity.
A further need exists for such a circuit which can be constructed on a single substrate and which is not affected by, nor produces, substrate noise injection.